U.S. patent number 7,995,877 [Application Number 12/182,683] was granted by the patent office on 2011-08-09 for optical nand gate.
This patent grant is currently assigned to Sandia Corporation. Invention is credited to James Raring, Erik J. Skogen, Anna Tauke-Pedretti.
United States Patent |
7,995,877 |
Skogen , et al. |
August 9, 2011 |
Optical NAND gate
Abstract
An optical NAND gate is formed from two pair of optical
waveguide devices on a substrate, with each pair of the optical
waveguide devices consisting of an electroabsorption modulator and
a photodetector. One pair of the optical waveguide devices is
electrically connected in parallel to operate as an optical AND
gate; and the other pair of the optical waveguide devices is
connected in series to operate as an optical NOT gate (i.e. an
optical inverter). The optical NAND gate utilizes two digital
optical inputs and a continuous light input to provide a NAND
function output. The optical NAND gate can be formed from III-V
compound semiconductor layers which are epitaxially deposited on a
III-V compound semiconductor substrate, and operates at a
wavelength in the range of 0.8-2.0 .mu.m.
Inventors: |
Skogen; Erik J. (Albuquerque,
NM), Raring; James (Goleta, CA), Tauke-Pedretti; Anna
(Albuquerque, NM) |
Assignee: |
Sandia Corporation
(Albuquerque, NM)
|
Family
ID: |
44350821 |
Appl.
No.: |
12/182,683 |
Filed: |
July 30, 2008 |
Current U.S.
Class: |
385/14 |
Current CPC
Class: |
B82Y
20/00 (20130101); G02F 1/01708 (20130101); H03K
19/017536 (20130101); G02B 2006/12078 (20130101); H01L
27/0605 (20130101); G02B 2006/12128 (20130101) |
Current International
Class: |
G02B
6/12 (20060101) |
Field of
Search: |
;385/14 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
S Kodama et al, "2.3 picoseconds optical gate monolithically
integrating photodiode and electroabsorption modulator",
Electronics Letters, vol. 37, No. 19, Sep. 13, 2001, pp. 1185-1186.
cited by other .
S. Kodama et al, "500 Gbit/s optical gate monolithically
integrating photodiode and electroabsorption modulator",
Electronics Letters, vol. 40, No. 9, Apr. 29, 2004, pp. 555-556.
cited by other .
David A. B. Miller et al, "The Quantum Well Self-Electrooptic
Effect Device: Optoelectronic Bistability and Oscillation, and
Self-Linearized Modulation," IEEE Journal of Quantum Electronics,
vol. QE-21, No. 9 Sep. 1985, pp. 1462-1476. cited by other .
James W. Raring et al, "Design and Demonstration of Novel QW
Intermixing Scheme for the Integration of UTC-Type Photodiodes with
QW-Based Components", IEEE Journal of Quantum Electronics, Feb.
2006, vol. 42, No. 2, pp. 171-181. cited by other .
James W. Raring et al, "40-Gb/s Widely Tunable Transceivers", IEEE
Journal of Selected Topics in Quantum Electronics, vol. 13, No. 1,
Jan.-Feb. 2007, pp. 3-14. cited by other .
Erik J. Skogen et al, "Monolithically Integrated Active Components:
A Quantum-Well Intermixing Approach", IEEE Journal of Selected
Topics in Quantum Electronics, vol. 11, No. 2, Mar./Apr. 2005, pp.
343-355. cited by other .
Toshihide Yoshimatsu et al, "100-Gb/s Error-Free Wavelength
Conversion with a Monolithic Optical Gate Integrating a Photodiode
and Electroabsorption Modulator", IEEE Photonics Technology
Letters, 2005, vol. 17, No. 11 pp. 2367-2369. cited by
other.
|
Primary Examiner: Le; Uyen-Chau N
Assistant Examiner: Tran; Hoang
Attorney, Agent or Firm: Hohimer; John P.
Government Interests
GOVERNMENT RIGHTS
This invention was made with Government support under Contract No.
DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The
Government has certain rights in the invention.
Claims
What is claimed is:
1. An optical NAND gate which receives a pair of digital optical
inputs each with a logic state and generates therefrom a digital
optical output which is a NAND representation of the logic states
of the pair of digital optical inputs, the optical NAND gate
comprising: a substrate; an optical AND gate formed on the
substrate and comprising a first electroabsorption modulator and a
first waveguide photodetector, with the first electroabsorption
modulator receiving a first digital optical input of the pair of
digital optical inputs, and with the first waveguide photodetector
receiving a second digital optical input of the pair of digital
optical inputs and generating therefrom a photocurrent signal, and
with the photocurrent signal being connected to modulate an
absorption of the first electroabsorption modulator through which
the first digital optical input is transmitted and thereby convert
the pair of digital optical inputs into a logical AND function
output of the first electroabsorption modulator; and an optical NOT
gate formed on the substrate and comprising a second
electroabsorption modulator and a second waveguide photodetector,
with the with the second waveguide photodetector receiving the
logical AND function output from the first electroabsorption
modulator and generating therefrom another photocurrent signal
which is electrically connected to modulate the absorption of the
second electroabsorption modulator which receives an input of
continuous light, thereby converting the continuous light input
into the digital optical output of the optical NAND gate.
2. The apparatus of claim 1 wherein the first electroabsorption
modulator and the first waveguide photodetector are electrically
connected together in parallel and are electrically connected
through a resistor to a bias voltage.
3. The apparatus of claim 1 wherein the second electroabsorption
modulator and the second waveguide photodetector are electrically
connected in series.
4. The apparatus of claim 1 wherein the input of the continuous
light is provided by a laser located on the substrate.
5. The apparatus of claim 4 wherein the laser comprises a
distributed-Bragg reflector (DBR) laser.
6. The apparatus of claim 1 further comprising a plurality of
optical waveguides on the substrate to guide the first digital
optical input to the first electroabsorption modulator, to guide
the second digital optical input to the first waveguide
photodetector, to guide the continuous light input to the second
electroabsorption modulator, to guide the logical AND function
output to the second waveguide photodetector, and to guide the
digital optical output from the second electroabsorption
modulator.
7. The apparatus of claim 1 wherein the substrate comprises a III-V
compound semiconductor substrate, and each electroabsorption
modulator and each waveguide photodetector comprises a plurality of
III-V compound semiconductor layers which are epitaxially grown on
the III-V compound semiconductor substrate.
8. The apparatus of claim 7 wherein the III-V compound
semiconductor substrate comprises indium phosphide (InP), and the
plurality of III-V compound semiconductor layers are selected from
the group consisting of indium gallium arsenide phosphide (InGaAsP)
layers, indium gallium arsenide (InGaAs) layers, indium aluminum
gallium arsenide (InAlGaAs) layers, and combinations thereof.
9. The apparatus of claim 7 wherein the III-V compound
semiconductor substrate comprises gallium arsenide (GaAs), and the
plurality of III-V compound semiconductor layers are selected from
the group consisting of GaAs layers, aluminum gallium arsenide
(AlGaAs) layers, indium gallium arsenide phosphide (InGaAsP)
layers, indium gallium arsenide (InGaAs) layers, and combinations
thereof.
10. The apparatus of claim 1 wherein the first digital optical
input and the second digital optical input have a wavelength in the
range of 0.8-2.0 microns.
11. An optical NAND gate which receives a first digital optical
input and a second digital optical input and generates therefrom a
NAND function digital optical output, comprising: a III-V compound
semiconductor substrate having a plurality of III-V compound
semiconductor layers epitaxially grown thereon; a first
electroabsorption modulator formed from the plurality of III-V
compound semiconductor layers, with the first electroabsorption
modulator receiving the first digital optical input; a first
waveguide photodetector formed from the plurality of III-V compound
semiconductor layers to receive the second digital optical input
and to generate therefrom a first photocurrent signal which changes
an absorption of light in the first electroabsorption modulator and
thereby converts the first digital optical input into an output of
the first electroabsorption modulator; a second waveguide
photodetector formed from the plurality of ITT-V compound
semiconductor layers to receive the output from the first
electroabsorption modulator and to generate therefrom a second
photocurrent signal; a laser formed from the plurality of III-V
compound semiconductor layers to provide a source of continuous
light; and a second electroabsorption modulator formed from the
plurality of III-V compound semiconductor layers to receive the
continuous light from the laser and to convert the continuous light
into the NAND function digital optical output in response to the
second photocurrent signal which is electrically connected to vary
the absorption of the continuous light being transmitted through
the second electroabsorption modulator.
12. The apparatus of claim 11 wherein the III-V compound
semiconductor substrate comprises indium phosphide (InP), and the
plurality of III-V compound semiconductor layers are selected from
the group consisting of indium gallium arsenide phosphide (InGaAsP)
layers, indium gallium arsenide (InGaAs) layers, indium aluminum
gallium arsenide (InAlGaAs) layers, and combinations thereof.
13. The apparatus of claim 11 wherein the III-V compound
semiconductor substrate comprises gallium arsenide (GaAs), and the
plurality of III-V compound semiconductor layers are selected from
the group consisting of GaAs layers, aluminum gallium arsenide
(AlGaAs) layers, indium gallium arsenide phosphide (InGaAsP)
layers, indium gallium arsenide (InGaAs) layers, and combinations
thereof.
14. The apparatus of claim 11 wherein the first electroabsorption
modulator and the first waveguide photodetector are electrically
connected in parallel, and are further connected through a resistor
to a bias voltage.
15. The apparatus of claim 14 wherein the second electroabsorption
modulator and the second waveguide photodetector are electrically
connected in series, and are further connected to at least one
additional bias voltage.
16. The apparatus of claim 11 wherein the laser comprises a
distributed Bragg reflector (DBR) laser.
17. The apparatus of claim 11 wherein a plurality of passive
optical waveguides are provided on the substrate and formed from
the plurality of III-V compound semiconductor layers to guide the
first digital optical input to the first electroabsorption
modulator, to guide the second digital optical input to the first
waveguide photodetector, to guide the continuous light from the
laser to the second electroabsorption modulator, to guide the
output from the first electroabsorption modulator to the second
waveguide photodetector, and to guide the NAND function digital
optical output from the second electroabsorption modulator.
18. The apparatus of claim 11 wherein the first digital optical
input and the second digital optical input have a wavelength in the
range of 0.8-2.0 microns.
19. A photonic integrated circuit which generates a NAND function
digital optical output from a pair of digital optical inputs,
comprising: a first pair of optical waveguide devices formed on a
substrate, with the first pair of optical waveguide devices
comprising a first electroabsorption modulator which is
electrically connected in parallel with a first photodetector, with
the first electroabsorption modulator receiving a first digital
optical input of the pair of digital optical inputs, and with the
first photodetector receiving a second digital optical input of the
pair of digital optical inputs to generate therefrom a photocurrent
signal which changes a reverse-bias voltage on the first
electroabsorption modulator to generate a digital optical output of
the first electroabsorption modulator; and a second pair of optical
waveguide devices formed on the substrate, with the second pair of
optical waveguide devices comprising a second electroabsorption
modulator which is electrically connected in series with a second
photodetector, with the second electroabsorption modulator
receiving an input of continuous light, and with the second
photodetector receiving the digital optical output from the first
electroabsorption modulator and generating therefrom another
photocurrent signal which changes the reverse-bias voltage on the
second electroabsorption modulator to modulate the continuous light
being transmitted through the second electroabsorption modulator
and thereby generate the NAND function digital optical output.
20. The apparatus of claim 19 wherein the continuous light input is
provided by a laser located on the substrate.
Description
FIELD OF THE INVENTION
The present invention relates in general to digital optical logic
gates, and in particular to a optical NAND gate which utilizes
electroabsorption modulators and waveguide photodetectors to
generate a NAND function digital optical output from a pair of
digital optical inputs.
BACKGROUND OF THE INVENTION
Optical logic gates have been the subject of research for several
decades due to the possibility of achieving higher operating speeds
than logic based on electronics. The advantages of digital signal
processing in the optical domain include higher signal bandwidth,
lower signal cross-talk, and greater protection against electronic
eavesdropping. All-optical signal processing also eliminates the
need to convert signals from the optical domain into the electronic
domain for processing and then to re-convert the processed signals
from the electronic domain back into the optical domain. This can
reduce the cost, electrical power requirement, size and weight
needed for optical-to-electronic converters, electronic signal
processing circuitry, and electronic-to-optical converters.
The present invention addresses the need for optical logic gates by
providing an optical NAND gate which can be formed as a photonic
integrated circuit (PIC) with two electroabsorption modulator (EAM)
photodiode (PD) pairs, with a first EAM/PD pair being electrically
connected in parallel to operate as an optical AND gate, and with a
second EAM/PD pair being electrically connected in series to
operate as an optical NOT gate. This configuration according to the
present invention provides advantages in terms of optical isolation
of input and output signals, an ability to be monolithically
integrated and an ability to operate using direct-current
electrical power sources with a relatively low power consumption
and a relatively compact size. The present invention is also
advantageous in providing for optical signal gain and regeneration
thereby permitting a fan out capability which can allow multiple
optical NAND gates to be interconnected together to provide a
higher level of logic functionality as needed for optical signal
processing or optical computing.
These and other advantages of the present invention will become
evident to those skilled in the art.
SUMMARY OF THE INVENTION
The present invention relates to an optical NAND gate which
receives a pair of digital optical inputs each with a logic state
and generates therefrom a digital optical output which is a NAND
representation of the logic states of the pair of digital optical
inputs. The optical NAND gate comprises a substrate; an optical AND
gate formed on the substrate and comprising a first
electroabsorption modulator and a first waveguide photodetector,
and an optical NOT gate (i.e. an inverter) formed on the substrate
and comprising a second electroabsorption modulator and a second
waveguide photodetector. The first electroabsorption modulator
receives a first digital optical input of the pair of digital
optical inputs. The first waveguide photodetector receives a second
digital optical input of the pair of digital optical inputs and
generates therefrom a photocurrent signal. This photocurrent signal
is connected to modulate an absorption of the first
electroabsorption modulator through which the first digital optical
input is transmitted and thereby convert the pair of digital
optical inputs into a logical AND function output of the first
electroabsorption modulator (i.e. an AND representation of the
logic states of the pair of digital optical inputs).
The second waveguide photodetector receives the logical AND
function output from the first electroabsorption modulator and
generates therefrom another photocurrent signal. This other
photocurrent signal is electrically connected to modulate the
absorption of the second electroabsorption modulator which receives
an input of continuous light, thereby converting the continuous
light input into the digital optical output of the optical NAND
gate.
The first electroabsorption modulator and the first waveguide
photodetector are electrically connected together in parallel, and
are electrically connected through a resistor to a bias voltage.
The second electroabsorption modulator and the second waveguide
photodetector are electrically connected in series.
The continuous light input can be generated by a laser which can be
located on the substrate. The laser can comprise a
distributed-Bragg reflector (DBR) laser. The laser can operate in
the range of 0.8-2.0 microns, with the pair of digital optical
inputs also being in this same wavelength range.
A plurality of optical waveguides can be provided on the substrate
to guide the first digital optical input to the first
electroabsorption modulator, to guide the second digital optical
input to the first waveguide photodetector, to guide the continuous
light input to the second electroabsorption modulator, to guide the
logical AND function output to the second waveguide photodetector,
and to guide the digital optical output from the second
electroabsorption modulator.
The substrate can comprise a III-V compound semiconductor
substrate. Each electroabsorption modulator and each waveguide
photodetector can comprise a plurality of III-V compound
semiconductor layers which are epitaxially grown on the III-V
compound semiconductor substrate.
In certain embodiments of the present invention, the III-V compound
semiconductor substrate can comprise indium phosphide (InP); and
the plurality of III-V compound semiconductor layers can be
selected from the group consisting of indium gallium arsenide
phosphide (InGaAsP) layers, indium gallium arsenide (InGaAs)
layers, indium aluminum gallium arsenide (InAlGaAs) layers, and
combinations thereof. In other embodiments of the present
invention, the III-V compound semiconductor substrate can comprise
gallium arsenide (GaAs), and the plurality of III-V compound
semiconductor layers can be selected from the group consisting of
GaAs layers, aluminum gallium arsenide (AlGaAs) layers, InGaAsP
layers, InGaAs layers, and combinations thereof.
The present invention further relates to an optical NAND gate which
receives a first digital optical input and a second digital optical
input and generates therefrom a NAND function digital optical
output. The optical NAND gate comprises a III-V compound
semiconductor substrate having a plurality of III-V compound
semiconductor layers epitaxially grown thereon; a first
electroabsorption modulator formed from the plurality of III-V
compound semiconductor layers, with the first electroabsorption
modulator receiving the first digital optical input; a first
waveguide photodetector formed from the plurality of III-V compound
semiconductor layers to receive the second digital optical input
and to generate therefrom a first photocurrent signal which changes
an absorption of light in the first electroabsorption modulator and
thereby converts the first digital optical input into an output of
the first electroabsorption modulator; a second waveguide
photodetector formed from the plurality of III-V compound
semiconductor layers to receive the output from the first
electroabsorption modulator and to generate therefrom a second
photocurrent signal; a laser formed from the plurality of III-V
compound semiconductor layers to provide a source of continuous
light; and a second electroabsorption modulator formed from the
plurality of III-V compound semiconductor layers to receive the
continuous light from the laser and to convert the continuous light
into the NAND function digital optical output in response to the
second photocurrent signal which is electrically connected to vary
the absorption of the continuous light being transmitted through
the second electroabsorption modulator. The first digital optical
input and the second digital optical input can each have a
wavelength in the range of 0.8-2.0 microns.
In some embodiments of the present invention, the III-V compound
semiconductor substrate can comprise indium phosphide (InP); and
the plurality of III-V compound semiconductor layers can be
selected from the group consisting of indium gallium arsenide
phosphide (InGaAsP) layers, indium gallium arsenide (InGaAs)
layers, indium aluminum gallium arsenide (InAlGaAs) layers, and
combinations thereof. In other embodiments of the present
invention, the III-V compound semiconductor substrate can comprise
gallium arsenide (GaAs); and the plurality of III-V compound
semiconductor layers can be selected from the group consisting of
GaAs layers, aluminum gallium arsenide (AlGaAs) layers, indium
gallium arsenide phosphide (InGaAsP) layers, indium gallium
arsenide (InGaAs) layers, and combinations thereof. A plurality of
passive optical waveguides can be provided on the substrate and
formed from the plurality of III-V compound semiconductor layers.
These passive optical waveguides can be used to guide the first
digital optical input to the first electroabsorption modulator, to
guide the second digital optical input to the first waveguide
photodetector, to guide the continuous light from the laser to the
second electroabsorption modulator, to guide the output from the
first electroabsorption modulator to the second waveguide
photodetector, and to guide the NAND function digital optical
output from the second electroabsorption modulator.
The first electroabsorption modulator and the first waveguide
photodetector can be electrically connected in parallel, and can be
connected through a resistor to a bias voltage. The second
electroabsorption modulator and the second waveguide photodetector
can be electrically connected in series, and can be connected to at
least one additional bias voltage. The laser can comprise a
distributed Bragg reflector (DBR) laser.
The present invention also relates to a photonic integrated circuit
(PIC) which generates a NAND function digital optical output from a
pair of digital optical inputs. The PIC comprises a first pair of
optical waveguide devices formed on a substrate, with the first
pair of optical waveguide devices comprising a first
electroabsorption modulator which is electrically connected in
parallel with a first photodetector, with the first
electroabsorption modulator receiving a first digital optical input
of the pair of digital optical inputs, and with the first
photodetector receiving a second digital optical input of the pair
of digital optical inputs to generate therefrom a photocurrent
signal which changes a reverse-bias voltage on the first
electroabsorption modulator to generate a digital optical output of
the first electroabsorption modulator; and a second pair of optical
waveguide devices formed on the substrate, with the second pair of
optical waveguide devices comprising a second electroabsorption
modulator which is electrically connected in series with a second
photodetector, with the second electroabsorption modulator
receiving an input of continuous light, and with the second
photodetector receiving the digital optical output from the first
electroabsorption modulator and generating therefrom another
photocurrent signal which changes the reverse-bias voltage on the
second electroabsorption modulator to modulate the continuous light
being transmitted through the second electroabsorption modulator
and thereby generate the NAND function digital optical output. The
continuous light input can be provided by a laser located on the
substrate.
Additional advantages and novel features of the invention will
become apparent to those skilled in the art upon examination of the
following detailed description thereof when considered in
conjunction with the accompanying drawings. The advantages of the
invention can be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and form a
part of the specification, illustrate several aspects of the
present invention and, together with the description, serve to
explain the principles of the invention. The drawings are only for
the purpose of illustrating preferred embodiments of the invention
and are not to be construed as limiting the invention. In the
drawings:
FIG. 1 shows a schematic plan view of a first example of the
optical NAND gate of the present invention.
FIG. 2A shows a schematic diagram of an electrical and optical
circuit of a parallel-connected first waveguide photodetector and
first electroabsorption modulator from the optical NAND gate of
FIG. 1.
FIG. 2B shows a logical truth table for the electrical and optical
circuit of FIG. 2A which functions as an optical AND gate.
FIG. 2C shows a schematic diagram of an electrical and optical
circuit of a series-connected second waveguide photodetector and
second electroabsorption modulator from the optical NAND gate of
FIG. 1.
FIG. 3A shows a schematic diagram of the electrical and optical
circuit for the entire optical NAND gate of FIG. 1.
FIG. 3B shows a logical truth table for the optical NAND gate of
FIG. 1.
FIGS. 4A-4H show schematic cross-section views along the section
lines 1-1 and 2-2 in FIG. 1 to illustrate a series of process steps
in the fabrication of the optical NAND gate of FIG. 1. FIGS. 4A-4G
are taken along the section line 1-1 in FIG. 1; and FIG. 4H is
taken along the section line 2-2 in FIG. 1.
FIG. 5 shows a schematic plan view of a second example of the
optical NAND gate of the present invention with the continuous
light input being provided by an external laser.
FIG. 6 shows a schematic plan view of a third example of the
optical NAND gate of the present invention.
FIG. 7 shows a schematic diagram of the electrical and optical
circuit for a series-connected second waveguide photodetector and
second electroabsorption modulator in the optical NAND gate of FIG.
6.
FIG. 8 shows a schematic diagram of the electrical and optical
circuit for the entire optical NAND gate of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, there is shown a schematic plan view of a
first example of the optical NAND gate 10 of the present invention
which can be formed as a photonic integrated circuit (PIC). The
optical NAND gate 10 comprises a substrate 12 on which are formed a
first electroabsorption modulator (EAM) 14 and a first waveguide
photodetector (PD) 16 which are connected to function as an optical
AND logic gate (also referred to as an AND gate). Each
electroabsorption modulator and waveguide photodetector described
herein can be formed either as a lumped-element device, or as a
traveling-wave device.
In FIG. 1, the first electroabsorption modulator 14 receives a
first digital optical input 100, which is denoted as the "A" input.
This "A" input 100 can be directed to the first electroabsorption
modulator 14 via a passive optical waveguide 18 (also referred to
herein as an optical waveguide 18). The first waveguide
photodetector 16 in FIG. 1 receives a second digital optical input
110, which is denoted as the "B" input, with the "B" input 110
being directed to the first waveguide photodetector 16 using
another optical waveguide 18. The "B" input 110 is absorbed within
the first waveguide photodetector 16 to generate a photocurrent
signal which can be used to modulate an absorption of the "A" input
100 which is transmitted through the first electroabsorption
modulator 14. Since the "B" input is a digital signal, then the
photocurrent signal will also generally be a digital signal
depending upon a data rate for operation of the device 10. This
modulation, which changes the absorption of the modulator 14 in
response to a digital state of the photocurrent signal from the
first waveguide photodetector 16, converts the "A" input 100 into a
logical AND function output 120 of the first electroabsorption
modulator 14. Any of the "B" input 110 which is not absorbed in the
waveguide photodetector 16 to generate the photocurrent signal is
coupled into an L-shaped passive waveguide 20 which serves as an
optical trap.
The first electroabsorption modulator 14 and the first waveguide
detector 16 are electrically connected in parallel by wiring 22 on
the substrate 12. The wiring 22, which can be in the form of a
radio-frequency (rf) transmission line, connects an upper electrode
24 of the first electroabsorption modulator 14 to an upper
electrode 26 of the first waveguide detector 16. Additional wiring
22 connects a lower electrode 24' of the first electroabsorption
modulator 14 to a lower electrode 26' of the first waveguide
detector 16. The parallel-connected modulator 14 and photodetector
16 are further connected through a resistor 28 to a first bias
voltage V.sub.1 which is used to reverse bias both the modulator 14
and photodetector 16. The first bias voltage V.sub.1 can be a few
volts (e.g. -5 volts) and can be supplied by a direct-current (dc)
power supply which can be located off of the substrate 12 and
connected to the wiring 22 using a pair of bond pads 30 on the
substrate 12.
FIG. 2A shows a schematic diagram of the electrical and optical
circuit which is produced by the parallel-connected modulator 14
and photodetector 16 to illustrate operation of this first pair of
optical waveguide devices in the optical NAND gate 10. By design,
the photocurrent signal generated by the first waveguide
photodetector 16 in response to incident light (i.e. the "B" input
110) is relatively independent of an electric field produced
therein by the applied reverse-bias voltage V.sub.1 and depends
only upon the intensity of the incident light. This is also the
case for a second waveguide photodetector 32 which will be
described hereinafter. On the other hand, the absorption within the
first electroabsorption modulator 14 depends upon the electric
field produced herein by the applied reverse-bias voltage V.sub.1.
Thus, as the amount of the reverse-bias voltage V.sub.1 across the
electroabsorption modulator 14 increases, the absorption of light
therein will also increase either due to a Franz-Keldysh effect or
due to a quantum-confined Stark effect. This is also the case for a
second electroabsorption modulator 36 which will be described
hereinafter.
In FIG. 2A, when the "B" input 110 provides a very low or
nonexistent light level corresponding to a logical "0" state, no
photocurrent signal will be generated within the first waveguide
photodetector 16, so that no change in the reverse-bias voltage
across the photodetector 16 and modulator 14 will occur. In this
case, substantially the entire reverse-bias voltage V.sub.1 will be
applied across both the photodetector 16 and modulator 14. This
relatively high reverse-bias voltage V.sub.1 will result in a
relatively high electric field within the electroabsorption
modulator 14 to provide a high absorption of light therein so that
very little, if any, of the "A" input 100 will be transmitted
through the modulator 14 to form the light output 120 (also
referred to herein as a logical AND function output 120, or simply
as an output 120). Thus, when the "B" input is in the logical "0"
state, the output 120 will also be in the logical "0" state
regardless of the logical state of the "A" input.
If on the other hand, the "B" input 110 is at a relatively high
light level corresponding to a logical "1" state, then the light
from the "B" input 110 will be detected to generate the
photocurrent signal in the first waveguide photodetector 16. This
will produce an increased current flow through the resistor 30
which will reduce the amount of the reverse-bias voltage V.sub.1
which is dropped across the photodetector 16 and modulator 14. The
reduction in the amount of the reverse-bias voltage V.sub.1 across
the electroabsorption modulator 14 will reduce the electric field
therein so that the absorption of the "A" input 100 in the
modulator 14 will be reduced. If the "A" input 100 is in a logical
"1" state, the reduced absorption will allow substantially all of
the "A" input 100 to be transmitted through the modulator 14 to
provide a logical "1" state for the light output 120. When the "A"
input 100 corresponds to a logical "0" state, the output 120 from
the modulator 14 will also be in a logical "0" state when the "B"
input 110 is high (i.e. in a logical "1" state).
A logical truth table for operation of the electrical and optical
circuit of FIG. 2A is shown in FIG. 2B. This portion of the optical
NAND gate 10, which includes the parallel-connected modulator 14
and photodetector 16 functions as an optical AND gate, with the
light output 120 being defined by ANDing the "A" input 100 and the
"B" input 110.
Returning to FIG. 1, the light output 120 is guided out from the
first electroabsorption modulator 14 through another passive
waveguide 18 to a second waveguide photodetector 32 which includes
an upper electrode 34 and a lower electrode 34'. This second
waveguide photodetector 32 is electrically connected in series to a
second electroabsorption modulator 36, which has an upper electrode
38 and a lower electrode 38', using wiring 22. A second bias
voltage V.sub.2 can be provided to reverse-bias the
series-connected modulator 36 and photodetector 32. This can be
done using the same or a different dc power which supplies the
voltage V.sub.1, with each reverse bias voltage being up to a few
volts (e.g. -5 V). As shown in FIG. 1, the orientation of the upper
and lower electrodes for the second waveguide photodetector 32 and
the second electroabsorption modulator 36 can be reversed from that
of the first waveguide photodetector 16 and the first
electroabsorption modulator 14 to facilitate connecting the
photodetector 32 and modulator 36 in series.
The light output 120 which enters the second waveguide
photodetector 32 is absorbed therein to generate another
photocurrent signal, with any of the unabsorbed light output 120
which is transmitted through the photodetector 32 being trapped by
another L-shaped passive waveguide 20. The photocurrent signal
generated by the second waveguide photodetector 32 is used to
modulate the absorption of light in the second electroabsorption
modulator 36 since the series connection of the photodetector 32
and modulator 36 requires that the same photocurrent signal flow
through both of these waveguide devices 32 and 36 and this will
change the amount of the reverse-bias voltage V.sub.2 which is
dropped across the modulator 36.
Continuous light 130 (i.e. continuous lasing emission) from a laser
40 is directed through a passive waveguide 18 to the second
electroabsorption modulator 36 and is transmitted therethrough
depending upon the absorption of the modulator 36. The laser 40 can
be a distributed Bragg reflector (DBR) laser 40 which is located on
the substrate 12 with a plurality of electrodes including an upper
electrode 42 and a lower electrode 42'. The DBR laser 40 is
forward-biased with a current source Ito generate the continuous
light 130. The continuous light 130 from the DBR laser 40 can have
an optical power in the range of 3-10 milliWatts (mW) or more. In
other embodiments of the present invention, the continuous light
130 can be provided by a laser 40 which is located off the
substrate 12 (see FIG. 5) and coupled into the passive waveguide 18
on the substrate 12 using either free-space or optical fiber
coupling.
FIG. 2C shows a schematic diagram of the electrical and optical
circuit of the series-connected modulator 36 and photodetector 32
to illustrate operation of this second pair of optical waveguide
devices. The series-connected modulator 36 and photodetector 32
functions as a NOT logic gate (also referred to as a NOT gate, or
an optical inverter) using inputs of the continuous light 130 from
the laser 40, and the output 120 from the first electroabsorption
modulator 14. When the output 120 from the first electroabsorption
modulator 14 is in a logical "1" state, light from the output 120
is absorbed within the second waveguide photodetector 32 to
generate the photocurrent signal therein; and this photocurrent
signal must also flow through the second electroabsorption
modulator 36 since the two waveguide devices 32 and 36 are
electrically connected in series. As a result of the increased
current flow through the modulator 36, the electric field therein
must also increase; and this requires the amount of the
reverse-bias voltage V.sub.2, which is dropped across the modulator
36, to increase. This, in turn, increases the absorption of light
in the electroabsorption modulator 36 so that substantially all of
the continuous light 130 is absorbed therein. With very little, if
any, of the continuous light 130 being transmitted through the
second electroabsorption modulator 36, a NAND output 140 from the
modulator 36 is in a logical "0" level (i.e. in a logic state which
is opposite that of the output 120 from the first electroabsorption
modulator 14 which is in a logical "1" state).
When the output 120 from the first electroabsorption modulator 14
is in a logical "0" state, very little, if any, light reaches the
second waveguide photodetector 32 so that very little, if any,
photocurrent signal is generated therein. In this case, the
reverse-bias current flowing through the second electroabsorption
modulator 36 must also be of the same magnitude as the photocurrent
signal from the photodetector 32; and this requires that the amount
of the reverse-bias voltage V.sub.2, which is dropped across the
electroabsorption modulator 36, to be small so that the absorption
of light in the modulator 36 is also small. This small absorption
of the modulator 36 allows substantially all of the continuous
light 130 to be transmitted through the modulator 36 to provide a
logical "1" state for the NAND output 140. Again, the logical state
of the NAND output 140 is exactly opposite that of the output 120
from the first electroabsorption modulator 14. Thus, the electrical
and optical circuit of FIG. 2C functions as an optical NOT
gate.
The combined electrical and optical circuits of FIGS. 2A and 2C are
shown in FIG. 3A. This corresponds to the full operation of the
optical NAND gate 10 of FIG. 1. FIG. 3B also shows a logical truth
table for the combined electrical and optical circuit of FIG.
3A.
Fabrication of the optical NAND gate 10 of FIG. 1 will now be
described with reference to FIGS. 4A-4G which show a series of
schematic cross-section views of the device 10 along the section
line 1-1 in FIG. 1 during various steps in the fabrication of the
optical NAND gate 10, and with reference to FIG. 4H which shows a
schematic cross-section view along the section line 2-2 in FIG. 1.
Fabrication of the optical NAND gate 10 of FIG. 1 will be described
using a quantum-well intermixing process using a plurality of III-V
compound semiconductor layers which are epitaxially grown on the
substrate 12 which is also preferably a III-V compound
semiconductor substrate (e.g. comprising indium phosphide or
gallium arsenide). The quantum-well intermixing process allows the
fabrication of many different photonic integrated circuit (PIC)
elements to be formed on the same substrate 12 in a manner similar
to that of semiconductor integrated circuit (IC) fabrication, while
allowing the various elements, which can include passive optical
waveguides, waveguide photodetectors, waveguide electroabsorption
modulators, lasers, and semiconductor optical amplifiers, to be
individually optimized.
Those skilled in the art will understand that the optical NAND gate
10 of the present invention can also be fabricated using other
types of III-V compound semiconductor fabrication methods which are
well-known in the art. These other types of III-V compound
semiconductor fabrication methods include butt-joint regrowth,
selective area growth, and the use of offset quantum wells and are
detailed in the following articles which are incorporated herein by
reference: E. Skogen et al., "Monolithically Integrated Active
Components: A Quantum-Well Intermixing Approach," IEEE Journal of
Selected Topics in Quantum Electronics, vol. 11, pp. 343-355,
March/April 2005; J. W. Raring et al., "40-Gb/s Widely Tunable
Transceivers," IEEE Journal of Selected Topics in Quantum
Electronics, vol. 13, pp. 3-14, January/February 2007.
FIG. 4A shows a schematic cross-section view of the plurality of
III-V compound semiconductor layers which can be initially
epitaxially grown on the substrate 12 in preparation for
fabricating the optical NAND gate 10 of the present invention. The
III-V compound semiconductor layers can comprise, for example,
indium phosphide (InP), indium gallium arsenide phosphide
(InGaAsP), indium gallium arsenide (InGaAs), indium aluminum
gallium arsenide (InAlGaAs) layers and combinations thereof when
the substrate 12 comprises InP. Alternately, the III-V compound
semiconductor layers can comprise gallium arsenide (GaAs), aluminum
gallium arsenide (AlGaAs), InGaAsP, InGaAs, and combinations
thereof when the substrate 12 comprises GaAs. The following
discussion will describe fabrication of the optical NAND gate 10
using an InP substrate 12 with InP, InGaAsP and InGaAs layers
thereon, but those skilled in the art will understand that the
various process steps described hereinafter can be applied with
minor modifications to an optical NAND gate 10 formed on a GaAs
substrate 12 with a combination of GaAs, AlGaAs, InGaAsP, and
InGaAs layers thereon. Those skilled in the art will also
understand that other III-V compound semiconductor materials can be
used for the substrate 12 and layers epitaxially grown thereon.
In FIG. 4A, the substrate 12 can comprise a semi-insulating
Fe-doped InP substrate 12. A plurality of III-V compound
semiconductor layers are epitaxially grown on the Fe-doped InP
substrate 12 by metal-organic chemical vapor deposition (MOCVD).
These III-V compound semiconductor layers are in order of epitaxial
growth: an InP buffer layer (not shown), an n-type InGaAs lower
contact layer 44; a lower cladding layer 46 of n-type-doped InP
which can be 1-2 .mu.m thick; a lower waveguide layer 48 of InGaAsP
which is undoped (i.e. not intentionally doped) and about 0.11
.mu.m thick with a composition selected to provide an energy
bandgap wavelength .lamda..sub.g=1.3 .mu.m; an undoped MQW region
50 which is about 0.11 .mu.m thick and comprises a plurality of
alternating quantum well (QW) layers 52 and barrier layers 54 of
InGaAsP, with the quantum well layers 52 being about 6.5 nanometers
(nm) thick and having an energy bandgap wavelength .lamda..sub.g
of, for example, 1.55 .mu.m as measured by photoluminescence, and
with the barrier layers 54 being about 8 nm thick and having an
energy bandgap wavelength .lamda..sub.g=1.3 .mu.m; a upper
waveguide layer 56 of undoped InGaAsP about 0.11 .mu.m thick with
.lamda..sub.g=1.3 .mu.m; an undoped InP etch stop layer 58 about 15
nm thick; an undoped InGaAsP etch stop layer 60 about 20 nm thick
with .lamda..sub.g=1.3 .mu.m; and an undoped InP implant buffer
layer 62 about 0.45 .mu.m thick. As an example, the QW layers 52
can have the semiconductor alloy composition
In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y with x=0.735 and y=0.840 to
provide the energy bandgap wavelength .lamda..sub.g=1.55 .mu.m; and
the barrier layers 54 can be formed of
In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y with x=0.735 and y=0.513. Those
skilled in the art will understand that the exact composition of
the layers 52 and 54 in the MQW region 50 and the compositions of
the other III-V compound semiconductor layers can be adjusted as
needed to provide predetermined values for energy bandgaps of these
layers, with the energy bandgap wavelength .lamda..sub.g of the QW
layers 52 generally being in the range of 0.8-2.0 .mu.m.
An ion implant mask (e.g. comprising silicon nitride about 0.5
.mu.m thick) can then be provided over the substrate 12 and III-V
compound semiconductor layers with openings at locations wherein
phosphorous ions are to be implanted into the InP implant buffer
layer 62 for use in selectively disordering the MQW region 50. The
locations where the waveguide photodetectors 16 and 32 and a gain
region of the laser 40 are to be formed will generally not have a
disordered MQW region 50 since the MQW region 50 is epitaxially
grown to optimize the performance of the photodetectors 16 and 32
and the gain region of the laser 40. The phosphorous ions can be
implanted into the InP implant buffer layer 62 at an ion energy of
about 100 keV and a dose of about 5.times.10.sup.14 cm.sup.-2 with
the substrate 12 being at a temperature of about 200.degree. C. The
implanted phosphorous ions produce point defects in the InP implant
buffer layer 62.
A rapid thermal annealing step can then be used to drive the point
defects down into the MQW region 50 to intermix the QW layers 52
and the barrier layers 54 at the interfaces therebetween. This
intermixing produces a blue-shift the energy bandgap wavelength in
the MQW region 50. The rapid thermal annealing step can be
performed at a temperature in the range of 630-700.degree. C. and
can be timed for a time interval from about one-half minute up to a
few tens of minutes to provide a predetermined energy bandgap
wavelength for the MQW region 50 which will depend upon the exact
elements of the optical NAND gate 10 being formed. To form the
electroabsorption modulators 14 and 36, a first rapid thermal
annealing step can be used to provide a few tens of nanometer
blue-shift in the energy bandgap wavelength of the MQW region 50
(e.g. to .lamda..sub.g .about.1.50 .mu.m) to reduce an absorption
loss therein in the absence of any reverse-bias voltage. This same
blue-shift is provided for the MQW region 50 for the passive
waveguides 18 and for distributed Bragg reflector (DBR) mirror
regions which are used to form an optical cavity for the DBR laser
40 and for the gain region and an optional phase control region
located within the optical cavity of the DBR laser 40. An
additional blue-shift will be provided in a subsequent thermal
annealing step for the passive waveguides 18 and the DBR mirror
regions to further increase the blue-shift therein (e.g. to
.lamda..sub.g .about.1.43 .mu.m) and thereby further reduce the
absorption for these elements of the optical NAND gate 10. The
blue-shift in the energy bandgap wavelength of the MQW region 50
can be determined using a laser-excited room-temperature
photoluminescence spectroscopy measurement.
After the first rapid thermal annealing step, the InP implant
buffer layer 62 can be removed above the electroabsorption
modulators 14 and 36 while leaving the layer 62 in place over the
passive waveguides 18 and DBR mirror regions. This can be done
using a wet etching step to etch away the layer 62, with the wet
etching being terminated upon reaching the InGaAsP etch stop layer
60. Removal of the InP implant buffer layer 62 above the
electroabsorption modulators 14 and 36 prevents any further
blue-shift in the MQW region 50 at these locations since it removes
the source of point defects necessary for quantum-well
intermixing.
A second rapid thermal annealing step can then be performed at
about the same temperature for up to a few minutes (e.g. 2-3
minutes) to provide further intermixing of the QW and barrier
layers 52 and 54 to produce an additional few tens of nanometers
blue-shift the energy bandgap of the MQW region 50 in the remaining
regions where the InP implant buffer layer 62 is still present.
This additional blue-shift in the energy bandgap of the MQW region
50 further reduces the absorption loss in the passive waveguides 18
and the DBR mirror regions of the optical NAND gate 10.
After the second rapid thermal annealing step is performed, the
remaining InP implant buffer layer 62 and the InGaAsP etch stop
layer 60 can be completely removed from the substrate 12 by wet
etching. This is schematically illustrated in the cross-section
view of FIG. 4B. Another etching step can then be used to etch a
corrugated grating structure down partway into the upper waveguide
layer 56 to form a DBR mirror at each end of the DBR laser 40. A
rear DBR mirror for the DBR laser 40 can be formed with a length
of, for example, 200 .mu.m, and a front DBR mirror in the DBR laser
40 can have a length of, for example, 10-50 .mu.m long. The gain
region of the DBR laser 40 can have a length of, for example,
200-500 .mu.m. When a separately-contacted phase control section is
to be provided in the DBR laser 40 to provide for tuning of the
wavelength of the continuous light 130, the phase control section
can have a length of, for example, 75 .mu.m and can be connected
through wiring 22 on the substrate 12 to a separate bond pad 30
(not shown in FIG. 1).
A blanket MOCVD regrowth can then be performed to epitaxially grow
an upper cladding layer 64 of p-type-doped InP which can be, for
example, 2.35 .mu.m thick followed by a cap layer 66 of
p-type-doped InGaAs about 0.2 .mu.m thick. This is shown in FIG.
4C. The p-type-doped InP upper cladding layer 64 in combination
with the n-type-doped lower cladding layer 46 form a semiconductor
p-i-n junction about the MQW region 50 and waveguide layers 48 and
56 which are undoped (i.e. intrinsic). This semiconductor p-i-n
junction is used for electrically-activated elements in the optical
NAND gate 10 including the waveguide photodetectors 16 and 32, the
electroabsorption modulators 14 and 36 and the gain region of the
laser 40 and any semiconductor optical amplifiers (if used).
In other embodiments of the present invention, an offset
quantum-well region can be epitaxially grown above the upper
waveguide layer 56. This can be useful to form the photodetectors
16 and 32 as uni-traveling carrier photodetectors, and can also be
useful to form semiconductor optical amplifiers (SOAs). The use of
an offset quantum-well region provides a lower confinement factor
than the quantum-well region 50 and thus can increase the
saturation power level for the photodetectors 16 and 32 and any
SOAs and also allow operation at higher frequencies. Further
details of the fabrication of photodetectors and SOAs using offset
quantum-well region can be found in an article by J. W. Raring et
al., "Design and Demonstration of Novel QW Intermixing Scheme for
the Integration of UTC-Type Photodiodes with QW-Based Components,"
IEEE Journal of Quantum Electronics, vol. 42, pp. 171-181, February
2006, which is incorporated herein by reference.
An etch mask (not shown) can be provided over the substrate 12 and
photolithographically patterned for use in etching down through the
InGaAs cap layer 66 and the InP upper cladding layer 64 as shown in
FIG. 4D. This defines an effective waveguide width of the various
elements 16, 18, 32, 36 and 40 which can be, for example, up to a
few microns or more (e.g. 1-3 .mu.m for the waveguides 18,
modulators 32 and 36 and laser 40; and 1-10 .mu.m wide for the
photodetectors 16 and 32 and any SOAs). The waveguide
photodetectors 16 and 32 can each have a length of, for example,
30-70 .mu.m; and can be straight (e.g. when the photodetectors 16
and 32 have the same width as the passive waveguides 18) or can be
tapered at one or both ends thereof (e.g. when the photodetectors
16 and 32 have a width that is larger than the width of the passive
waveguides 18 and 20). The electroabsorption modulators 14 and 36
can have a length of, for example, 100-300 .mu.m.
In FIG. 4E, one or more additional etching steps can be used to
etch down to the InGaAs lower contact layer 44 and partway into the
semi-insulating InP substrate 12.
In FIG. 4F, layers of silicon nitride 68 and benzocyclobutene (BCB)
70 can then be deposited over the substrate 12 about the
photodetectors 16 and 32, the modulators 14 and 36 and the laser
40, with openings at the locations where the upper and lower
electrodes will be formed. The BCB 70 can be tapered outside of the
photodetector 16 to allow the resistor 28 to be formed directly
over the silicon nitride layer 68 on the InP substrate 12 which is
useful for heat sinking of the resistor 28 to the InP substrate 12.
The silicon nitride layers 68 can each be about 0.1-0.2 .mu.m
thick.
In FIG. 4G, the lower and upper electrodes can be deposited. The
lower electrodes 24', 26', 34', 38' and 42' can comprise, for
example, a gold/germanium/nickel/gold (Au/Ge/Ni/Au) metallization
with an overall thickness of about 0.5 .mu.m; and the upper
electrodes 24, 26, 34, 38, 42 and the wiring 22 can be formed from
a titanium/platinum/gold (Ti/Pt/Au) metallization with an overall
thickness of about 1 .mu.m. The resistor 28 can be deposited as a
thin-film metal resistor (e.g. comprising tantalum nitride or
nichrome) with a resistance of, for example, 25-50 Ohms.
Adjacent elements of the optical NAND gate 10, which are not
optically connected are electrically isolated by etching down
partway into the semi-insulating InP substrate 12 as shown in FIG.
4G. However, this does not electrically isolate adjacent elements
of the optical NAND gate 10 which must be optically connected via
one of the passive waveguides 18. For these elements including the
first electroabsorption modulator 14 which is optically connected
to the second waveguide photodetector 32, and the laser 40 which is
optically connected to the second electroabsorption modulator 36,
the III-V compound semiconductor layers in a passive waveguide
region between these optically-connected elements can be ion
implanted. Hydrogen ions can be implanted at an energy of about 200
keV to electrically isolate regions 72 of the p-type-doped III-V
compound semiconductor layers; and helium ions can be implanted at
an energy of about 2 MeV to electrically isolate regions 74 of the
n-type-doped III-V compound semiconductor layers, with the
helium-implanted regions 74 extending down partway into the InP
substrate 12. The hydrogen-implanted regions 72 can also extend
around the electrically-active elements 14, 16, 32, 36 and 40 as
shown in FIG. 4H to improve the electrical characteristics of these
elements. The helium-implanted regions 74 are located where the
n-type-doped III-V compound semiconductor layers extend between the
first electroabsorption modulator 14 and the second waveguide
photodetector 32 as shown in FIG. 4H, and also between the laser 40
and the second electroabsorption modulator 36 in the completed
device 10. The ion implantation steps can be performed with
suitable masking to protect the electrically-active elements after
epitaxial growth of the InP layer 64 and the InGaAs cap layer 66 as
previously described with reference to FIG. 4C. FIG. 4H shows a
schematic cross-section view along the section line 2-2 in FIG. 1
to illustrate the locations of hydrogen-implanted regions 72 and
helium-implanted regions 74 in the completed device 10.
FIG. 5 shows a schematic plan view of a second example of the
optical NAND gate 10 of the present invention. This second example
of the present invention can be formed in a manner similar to that
previously described for the first example of the optical NAND gate
10 with reference to FIGS. 4A-4H except that there is no laser 40
on the substrate 12. The laser 40 can be located off the substrate
12 as shown in FIG. 5 with the continuous light 130 being coupled
from the laser 40 into a passive waveguide 18 on the substrate 12
using free-space or optical fiber coupling. This second example of
the optical NAND gate 10 of the present invention operates in the
same way as the first example which has been described
previously.
FIG. 6 shows a schematic plan view of a third example of the
optical NAND gate 10 of the present invention. In this example of
the present invention, the first pair of optical waveguide devices
(i.e. the waveguide photodetector 16 and the first
electroabsorption modulator 14) are electrically connected together
in parallel to form the electrical and optical circuit which has
been previously described with reference to FIG. 2A.
A semiconductor optical amplifier (SOA) 76 can be optionally
provided on the substrate 12 to amplify the "B" input 110 prior to
detection of the "B" input 110 with the first waveguide
photodetector 16 as shown in FIG. 6. This is useful to provide an
increased optical power (e.g. up to a few tens of milliWatts) for
the amplified "B" input 110, which will provide a larger
photocurrent signal from the first waveguide photodetector 16 and
thereby provide a larger on/off contrast in the light output 120
from the first electroabsorption modulator 14.
Another SOA 76 can be optionally provided to similarly amplify the
output 120 from the first electroabsorption modulator 14 as shown
in FIG. 6 to allow the generation of a larger photocurrent signal
from the second waveguide photodetector 32 and thereby provide a
larger on/off contrast ratio in the NAND output 140 from the second
electroabsorption modulator 36. This is also useful to reduce the
amount of optical power needed for the continuous light 130 from
the laser 40. If needed, another SOA 76 can be located between the
laser 40 and the second electroabsorption modulator 36 to amplify
the continuous light 130 which is input to the modulator 36 (not
shown in FIG. 6).
Each SOA 76 can be formed using a gain region similar to the gain
region in the laser 40 with an upper electrode 78 and a lower
electrode 78'. The gain region for each SOA 76 can have a length
of, for example, 100-500 .mu.m, and a width of, for example, 1-10
.mu.m. Alternatively, each SOA 76 can be formed as flared SOA 76
with a width that increases along the length of the SOA 76, or with
an offset multi-quantum-well (MQW) gain region. The use of a flared
SOA 76 or an SOA 76 having an offset MQW gain region is useful to
provide a higher saturation power level for the SOA 76.
In the example of FIG. 6, the laser 40 can be driven by a first
current source I.sub.1; and the SOAs 76 can be driven by additional
current sources I.sub.2 and I.sub.3 as shown in FIG. 6. Each
current source forward biases the gain region to generate optical
gain therein.
In the example of FIG. 6, a different electrical and optical
circuit from that of FIG. 2C is provided for the second pair of the
optical waveguide devices comprising the second waveguide
photodetector 32 and the second electroabsorption modulator 36.
This electrical and optical circuit configuration, which also
functions as an optical NOT gate, is schematically illustrated in
FIG. 7 and uses separate reverse-bias voltages V.sub.2 and V.sub.3
for the second electroabsorption modulator 36 and for the second
waveguide photodetector 32, respectively. The use of two separate
reverse-bias voltages V.sub.2 and V.sub.3 allows a resistor 80
(e.g. a 20-50 Ohm resistor) to be connected between the
photodetector 32 and the modulator 36 to ground. This allows a
higher reverse-bias voltage change to be produced on the modulator
36, thereby reducing the optical power required for the continuous
light 130 from the laser 40, and also providing a higher operating
speed for the optical NAND gate 10.
In the absence of any light output 120 incident on the second
waveguide photodetector 32, no photocurrent signal will be
generated so that node "C" where the photodetector 32, modulator 36
and the resistor 80 are all connected together will be at about
ground electrical potential. This drops the entire reverse-bias
voltage V.sub.2 (e.g. -1 volt) across the modulator 36. Since
V.sub.2 is relatively small, a relatively small absorption will be
produced within the modulator 36 so that substantially all of the
continuous light 130 will be transmitted through the modulator 36
to provide a high light level for the NAND output 140 corresponding
to a logical "1" state.
When the light output 120 is in a logical "1" state, the light
output 120 is amplified by the SOA 76 to provide a relatively high
optical power which can be, for example, up to a few tens of
milliWatts (e.g. 40 mW). This relatively high optical power of the
amplified light output 120 will generate a relatively large
photocurrent signal in the photodetector 32, with the photocurrent
signal flowing through resistor 80. The reverse-bias voltage
V.sub.3 (e.g. -5 volts) is also much larger than V.sub.2 so that
the photocurrent signal generated by the photodetector 32 will
produce a large change in the electrical potential at node "C"
which will add to and substantially increase the amount of
reverse-bias voltage which is dropped across the modulator 36. This
will greatly increase the absorption of the continuous light 130
within the modulator 36 so that very little, if any, of the
continuous light 130 will transmitted through the modulator 36 to
form the NAND output 140 which will then be in a logical "0" state
(i.e. in a logic state opposite that of the light output 120 from
the first electroabsorption modulator 14). Thus, the electrical and
optical circuit of FIG. 7 also acts as an optical NOT gate
providing a logic state for the NAND output 140 which is exactly
opposite of the logic state of the AND output 120.
FIG. 8 shows the complete schematic diagram of the electrical and
optical circuit for the third example of the optical NAND gate 10
of the present invention. This example of the optical NAND gate 10
has the same logical truth table as shown in FIG. 3B, and can be
fabricated using the process steps previously described with
reference to FIGS. 4A-4H.
Those skilled in the art will understand that the optical NAND gate
10 of the present invention as a building block to form other types
of optical logic gates, or to form an optical processor or optical
computer. A plurality of NAND gates 10 can be formed on a common
substrate 12 and optically connected with passive waveguides 18 to
form the other optical logic gates, or to form the optical
processor or optical computer in a way analogous to the
interconnection of a plurality of transistors to form an integrated
circuit. Thus, to form an optical signal processor or optical
computer, the NAND outputs 140 from a plurality of devices 10 would
be used to provide the "A" and "B" inputs for other devices 10 with
the exact number and optical interconnection of the various optical
NAND gates 10 depending upon the functionality of the optical
signal processor or optical computer.
The matter set forth in the foregoing description and accompanying
drawings is offered by way of illustration only and not as a
limitation. The actual scope of the invention is intended to be
defined in the following claims when viewed in their proper
perspective based on the prior art.
* * * * *